u COULD try making a invisible unit somewhere in the map that when a level starts takes damage and that makes your units run. something like:
event-Unit - unit's life becomes Less than 50.00
condition-none
action- create (number) units at (start region) for (neutral hostile) facing (180)
and than.
event- every 2 seconds of game time
action- Unit Group - Pick every unit in (Units in (region)) and do (Unit - Order (picked unit) to Move To (Center of (end region)))
and if they run round try removing attacks. aqasuision range or something like that

hwen they will attack to 0.
ill look around in my maps folder i should have a short tutorial map how to make a td. else try looking on wc3forums they have GREAT TUTS! (no im not advertising)